Effects of temperature and irradiance on the growth of the mixotrophic dinoflagellate Takayama helix: mixotrophy as a survival strategy

Increases in water temperature and exposure of photosynthetically active radiation due to global warming are known to affect growth of some marine photosynthetic organisms. Takayama spp. are phototrophic dinoflagellates and have caused fish kills. Recently, Takayama helix has been shown to be a mixotrophic species that can feed on diverse dinoflagellate prey species. We explored the effects of water temperature and light intensity, two important physical factors, on its autotrophic and mixotrophic growth rates. Both autotrophic and mixotrophic growth and ingestion rates on Alexandrium minutum were significantly affected by water temperature. The maximum autotrophic and mixotrophic growth rates of this species occurred at 28 and 26 ºC, respectively, but any cell of T. helix growing autotrophically or mixotrophically did not survive at 30 ºC. Therefore, cells of T. helix under unfavorable temperature conditions were not able to survive even when the prey was added. Both the autotrophic and mixotrophic growth rates and ingestion rates of T. helix on prey were significantly affected by photon flux density. At high photon flux density (i.e., ≥ 247 µmol photons m^-2 s^-1), the autotrophic growth rates were negative, but mixotrophy turned these negative rates to positive. Therefore, mixotrophy might be an effective strategy for T. helix to grow when it encounters high light intensity. Therefore, water temperature and light intensity affect on its autotrophic and mixotrophic growth rates, however, mixotrophy converts negative growth driven by high irradiance to positive, but not by high water temperature.